Since error messages are tagged by a line number it would be nice to have them in the editor. Don't know if this can be accomplished with factorio modding interface.

Totally can be, but I definitely won't bother.
I'd suggest using proper editor for code, not some wimpy in-game textbox, and just paste it here.

Oh also, it doesn't do that when window is open, but if you re-open it, error line should be highlighted in red.
Should probably make it highlight these immediately, but for parser bugs idk if it's worth the effort either, as textboxes being smart (and laggy, buggy, non-copy-pastable, etc) is one thing that annoys me personally to no end everywhere :)

Marking the error position is completely ok.
Your second sentence - I am not 100% sure what exactly you are talking about (my English is not good enough).
But if you want to say that it is difficult to accomplish with factorio then it is ok for me to leave it as it is - in case of parser errors.